This project is designed to provide a detailed understanding of those factors governing DNA flexibility. Such information is of central importance in helping to resolve the broader issue of how extremely long DNA molecules arrange themselves within such compact structures as virus particles and chromosomes. I propose to exploit the availability of recombinant DNA molecules of precisely defined sequence and length, in combination with two extremely sensitive hydrodynamic methods; namely, electric birefringence and intrinsic viscocity, in order to develop a self-consistent approach to the flexibility problem. In the first phase of this research, the dependence of the persistence length (P) of DNA on ionic strength, counterion valence, and specific counterion effects will be studied in order to determine the electrostatic contribution to P. The use of two distinct hydrodynamic approaches for the measurement to DNA flexibility under identical solution conditions will provide an important test of the consistency of both methods. In the second phase of this project, other factors which are thought to influence the conformation and flexibility of DNA will be studied. These include: intercalating agents, single-strand nicking, and base sequence. The temperature dependence of P will also be studied in an effort to obtain accurate values for the thermodynamic quantities governing DNA flexibility. The third phase of this research will involve a study of the electric-field-induced melting of DNA, since this approach holds promise as a means of probing the arrangement of counterions around the DNA molecule. Finally, I will attempt to estimate experimentally the magnitude and ionic strength dependence of excluded volume effects for DNA molecules of intermediate size, and will devote theoretical effort to the development of a model for the rigid cylinder for use in rotational diffusion studies.